1. Field of the Invention.
The present invention deals with multi-component adhesives and, in particular, with methods and apparatus for applying such adhesives.
2. Description of the Prior Art.
Various chemically reactive adhesives which are characterized by the occurrence of a chemical reaction during the formation of the adhesive bond are known in the art. A number of these chemically reactive adhesives are multi-component systems such as urea-formaldehyde, phenol-formaldehyde, resorcinol-formaldehyde or resorcinol-phenol-formaldehyde systems. In a resorcinol-phenol-formaldehyde system, for example, a viscous solution of a water soluble, fusible phenol-resorcinol-formaldehyde resin is first made. So that such a resin may be stored for an extended period of time without hardening, the amount of formaldehyde in this resin is limited. When, however, it is desired that an adhesive mix be formed from this resin in preparation for its application, a methylene donor, commonly referred to as a hardener, is added to the resin to cure it to a cross-linked insoluable, infusible state. Commonly used hardeners are liquids such as aqueous formaldehyde solutions or solids such as or hexamethylenetretramine. Solid hardeners are sometimes mixed with liquids to form a slurry and, for definitional purposes, the use herein of the term "liquid hardener" will encompass hardeners which are in a slurry as well as those hardeners which are in a solution.
It is often desirable that the resin and hardeners in a multi-component adhesive be mixed together as soon as possible before application so as to avoid the possibility that premature curing might occur. In U.S. Pat. No. 3,374,987 an apparatus is disclosed in which an adhesive applicator contains an integral device for mixing a liquid resin and a liquid hardener so that application and mixing occur substantially simultaneously. Although this applicator would appear to substantially preclude the possibility of premature curing of the adhesive, certain problems relating to the application of multicomponent adhesives still remain to be addressed. Specifically, it is known that changes in temperatures may result in changes in the viscosities of a liquid resin or a liquid hardener. In as much as a precise ratio of resin to hardener may be required to achieve certain desired characteristics in curing conditions or in the adhesive bond, it may be necessary for an operator to make numerous adjustments to the rates at which liquid resins and hardeners are introduced into the mixer-applicator so as to ensure that this precise ratio is maintained during conditions of changing temperature. Additionally, it may also be desired to increase the rate of flow of adhesive from the applicator. In order to achieve such an increase in adhesive flow rate with mixer-applicators heretofore known, it has been necessary for an operator to engage in the time consuming procedure of increasing the rate of flow of the resin and then increasing the hardener flow rate to the exact rate which would allow maintenance of the desired resin to hardener ratio. It is, therefore, the object of the present invention to provide a method and apparatus for applying a multi-component adhesive in which the mixing of liquid resins and liquid hardeners occurs substantially simultaneously with the application of the resulting adhesive and in which a precise resin to hardener ratio is maintained regardless of changes in temperature or in volumetric requirements for the adhesive.